1. Field of the Invention
The present invention relates generally to battery powered electrical devices, and, more particularly, to an improved battery charger integrated circuit ("IC") adapted for inclusion in portable electrical devices.
2. Decription of the Prior Art
A system has been specified for use in battery powered portable devices that is identified as the System Management Bus ("SMBus"). The SMBus prescribes data protocols, device addresses, and additional electrical requirements necessary to transport commands and information among various subsystems of a battery powered device. The SMBus specification envisions the SMBus interconnecting at least a system host computer, a smart battery charger, and a smart battery that are all included in the portable device. Under the SMBus protocol, the smart battery provides data, via the SMBus, to the portable device's host computer. A power management routine executed by the host computer processes such smart battery data to manage operation of at least the smart battery and the smart battery charger.
In accordance with the SMBus specification and protocol, a smart battery accurately reports its characteristics to the host computer via the SMBus. If a portable device includes more than one battery, each battery reports such characteristics independently via the SMBus. Providing the power management routine executed by the host computer with information about the charge state of each battery permits displaying the batteries' condition, and accurately estimating the portable device's remaining operating time. However, in addition to providing information about the batteries' charge state, the information obtained via the SMBus is sufficient to permit electrical power management for the portable device, and to also permit controlling battery charging regardless of a battery's particular chemistry.
To achieve the preceding objectives, the SMBus specifies that, independent of host computer power management routine operation, a smart battery charger must periodically poll a smart battery that is being charged for the battery's charging characteristics. Upon receiving a response from the smart battery, the smart battery charger then adjusts its output to match the smart battery's requirements. To avoid battery damage, the smart battery also reports certain conditions such as over charge, over voltage, over temperature, and too rapid temperature increase to the smart battery charger. In this way the smart battery effectively controls its re-charging cycle. Moreover, to prolong smart battery life, the smart battery charger may prevent a fully charged smart battery from powering the portable device if a source of external electrical power is available.
Analogously, the power management routine executed by the host computer may poll the smart battery, that powers the host computer's operation, for smart battery information. The power management routine can request factual information about the battery such as the battery's chemistry, or the battery's operating temperature, voltage, or charge or discharge current. The power management routine can then either display such information directly and/or to display an estimate of the battery's operating capabilities, or it may process such information for use in the computer system's power management scheme. Similar to the smart charger, the power management routine receives information about critical events if the smart battery detects a problem. Moreover, the power management routine also receives smart battery estimates about end of discharge, electrical capacity remaining below a preset threshold value, and time remaining until discharge below a preset threshold value.
As part of the host computer's power management scheme, the power management routine may provide other routines with information about battery condition. Accordingly, the power management routine may query a device driver routine to determine if an anticipated action will endanger the host computer's electrical power integrity. For example, before attempting to start a hard disk drive the power management routine may first determine if that particular operation might cause the smart battery's output voltage to drop below a threshold for host computer failure. Under such circumstances, the hard disk device driver's response might be to increase power available for starting the hard disk drive by causing the power management routine to turn-off a non-critical power consumption such as liquid crystal display ("LCD") backlighting.
In addition to a smart battery and a smart charger, a portable device that implements the SMBus will, in general, also include a smart battery selector. The SMBus specification and protocol includes a smart battery selector because a portable device may include two or more smart batteries, only one of which may be in use for powering the portable device's operation at any instant in time. In such multi-battery devices, the smart battery selector must arbitrate between or among batteries. Furthermore, the smart battery selector must be capable of swiftly re-configuring the portable devices power if a battery were to be suddenly removed, such as might occur if a battery were removed from a laptop or notebook computer to install a floppy diskette drive.
Additional, more detailed information about the SMBus specifications and protocol, and about smart batteries is provided by:
System Management Bus Specification, Revision 1.0, Intel, Corporation, Feb. 15, 1995; PA1 System Management Bus BIOS Interface Specification, Revision 1.0, Intel, Corporation, Feb. 15, 1995; PA1 Smart Battery Charger Specification Revision 1.0, Duracell Inc. and Intel Corporation, Jun. 27, 1996; PA1 Smart Battery Data Specification Revision 1.0, Duracell Inc. and Intel Corporation, Feb. 15, 1995; and PA1 Smart Battery Selector Specification Revision 1.0, Duracell Inc. and Intel Corporation, Sep. 5, 1996.
The publications listed above are hereby incorporated herein by reference as though fully set forth here.
U.S. patent application Ser. No. 08/850,335 filed May 2, 1997, entitled "Smart Battery Selector" describes a controller IC adapted for inclusion in a portable device. The portable device also includes at least two batteries that are capable of providing battery-state data via a bus to a host computer also included in the portable device. A control electronic-circuit included in the controller directs operation of switch-drivers for selecting among the batteries one of which powers operation of the portable device. A bus-snooper circuit allows the controller to monitor the bus for battery-condition alarm-messages independently of the host computer. The controller may respond to messages on the bus by independently selecting a different battery even if the host computer's operation has been suspended, perhaps to reduce power consumption. The controller disclosed in this patent may also independently select a single battery for charging, and may terminate charging upon receiving a battery overcharge message. The disclosure of the Smart Battery Selector patent application is hereby incorporated by reference.
While the SMBus specification and protocol addresses many significant problems associated with battery powered operation of portable devices, it omits details which are essential to address significant operational constraints involved in battery charging. For example, properly charging a battery requires continuously monitoring the charging current and, if necessary, adjusting operation of the battery charger so a prescribed charging current is supplied to the battery over time. U.S. Pat. No. 5,698,964 entitled "Adaptive Power Battery Charging Apparatus" ("the '964 patent") discloses a battery charger that includes a "buck converter circuit" battery charger having a feedback circuit which regulates battery charging. To permit regulating battery charging current, the circuit disclosed in this patent includes a current sensing resistor, depicted in FIG. 3, connected between a ground terminal of the battery being charged and circuit ground of the battery charger. During charging of the battery, current flowing through the current sensing resistor produces an electrical signal that is proportional to charging current. However, a difficulty encountered with such "low side" current sensing as that illustrated in the '964 patent is that it requires two electrically separate ground circuits, one for normal operation of the battery powered device, and another for battery charging.
U.S. Pat. No. 5,723,970 entitled "Battery Charging Circuitry Having Supply Current Regulation" ("the '970 patent") also discloses a battery charger that includes a feedback circuit which regulates battery charging. However, the feedback circuit disclosed in the '970 patent differs from that disclosed in the '964 patent by having the current sensing resistor located not between a battery ground and a charger ground, but rather between a source of battery charger's electrical energy and the battery being charged. Consequently, the circuit disclosed in the '970 patent is simpler than that disclosed in the '964 patent in the sense that it employs only a single ground that is common both to the battery powered device and to the charger. However, during battery charging the circuit disclosed in the '970 patent experiences a voltage at the "high-side" current sensing resistor which may exceed 16.8 volts ("V"), or may, if the battery is excessively discharged, be as low as 2.5 V. While circuits can be built using comparatively high voltage semiconductor devices or processes which are capable of accommodating this rather wide common-mode voltage range, it is difficult to envision a battery charger IC built using a conventional 5.0 V Complementary Metal-Oxide-Silicon ("CMOS") process that operates consistently throughout this large common-mode voltage range.